Saturday, May 1, 2010

Phoebe Nest is a Total Loss

On the morning of the 28th the Eastern Phoebe that we have on our webcam at work laid a third egg early in the morning. I grabbed this image on my way out the door at 6:53. It is hard to see as the camera was in infrared mode but there are three eggs in the nest.

When I got to work at 8:00, things had changed. There were only two eggs in the nest. The web cam caught this Brown Headed Cowbird stealing one of the phoebe eggs at 7:33.

This was sad to see but even more frustrating was when she came back the next day and destroyed the two remaining eggs. Here's what the nest looks like now.

There is just a little piece of broken shell. Cowbirds have gone after this nest in past years but they have always just added their eggs to the clutch and let the phoebe raise them. Cowbirds are brood parasites. They do not create their own nest, they lay their eggs in other birds nests and let the other birds raise their young.

A first I wondered if what we're seeing here could be an example of Mafia behavior. Research by Jeffrey P. Hoover and Scott K. Robinson shows that cowbirds will retaliate against other species for rejecting their eggs. The researchers showed that birds who rejected a cowbird egg had their own eggs destroyed 56% of the time while those who accepted them only had their eggs destroyed 6% of the time. This is actually a really interesting example of evolution in action. This behavior of the cowbirds helps drive evolution of other bird species in a direction beneficial to cowbirds. They are selecting for those birds who can't tell the cowbird eggs are in their nest while destroying the offspring (and genetic material) of those birds that have figured out what the cowbirds are up to.

This doesn't seem to be what happened in this nest though since it appears that the cowbirds just came in and destroyed the eggs having not laid any of their own first. This could be an example of what Hoover and Robinson call "farming." The cowbirds are trying to create more opportunities to parasitise the nest by making the phoebes start over. We'll see if they do.

I'm very conflicted about this. Nest parasitism is natural. I don't have a problem with it. However, the Brown-headed Cowbird is an edge species. It is a bird that historically hung out on the edge of the forest. It does not like the thick middle of a heavily wooded habitat and so those birds that do not live in edge areas have not had to compete with cowbirds. The problem is that humans simply adore edge. It is our favorite habitat probably owing to our own genetic heritage. We're slowly turning everywhere to edge as we open up forests while at the same time planting trees in open areas. We've allowed the cowbird into areas where it historically has not been and we've pet it into contact with species who have not had to deal with it in the past.

Cowbirds are just doing what they have always done, it is becoming a problem though because we've changed the playing field.

~Kirk

About the Site:

Kirk Mona is the founder of Twin Cities Naturalist.
He is a professional naturalist and writer.

I understand your conflicting viewpoints. I live near Fort Hood in Texas which contain the largest source population of the endangered Black-capped vireos and golden-cheeked warblers. Without cowbird traps, BCVs have insanely low nest success, but with the use of traps their populations have skyrocketed. It kinda sucks, but parasites are a part of nature.